Growth hormone is an anabolic hormone capable of promoting linear growth, weight gain and whole body nitrogen retention in animals. Growth hormone is produced in somatotroph cells of the anterior pituitary gland of animals and secreted throughout life. It is mainly controlled in the brain by two hypothalamic peptides: growth hormone releasing hormone (GHRH), which stimulates its secretion and synthesis; and somatostatin, which inhibits them. Growth hormone is known to promote skeletal growth, to increase nitrogen retention and protein synthesis, and to affect glucose and lipid metabolism in animals.
As animal grows older, the growth hormone undergoes considerable decrement resulting in a loss of skeletal muscle mass, osteoporosis, increased fat deposition and decreased lean body mass. It has been demonstrated that the development of these changes can be offset by recombinant growth hormone therapy. However, current recombinant growth hormone therapy has several shortcomings including frequent subcutaneous or intravenous injections, insulin resistance and impaired glucose tolerance.
Growth hormone has been administered to animals to offset the growth hormone deficiency in aged animals. Furthermore, there are reports of using growth hormone to boost certain desirable performances of animals. For example, growth hormone has been used to enhance growth performance in animals by increasing lean body mass with an overall improvement in feed efficiency, and to enhance fertility in animals. Additionally, growth hormone has been used to stimulate milk production and achieve an enhanced milk yield in mammals.
U.S. Pat. No. 4,521,409, issued on Jun. 4, 1985 to Bauman, and U.S. Pat. No. 5,004,728, issued on Apr. 2, 1991 to Chalupa disclose the administration of growth hormone to cattle and ruminants to stimulate milk production with an increase in feed to milk conversion. In U.S. Pat. No. 5,416,073, issued on May 16, 1995 to Coy, growth hormone is used to increase milk production in dairy cows and to increase growth rates in animals such as cattle, sheep, and swine.
U.S. Pat. No. 5,008,244, issued on Apr. 16, 1991 to Miller discloses an administration of growth hormone to food-producing animals to increase their fertility by improving embryonic survival and litter size during the reproductive stage of growth. In U.S. Pat. No. 5,583,106, issued on Dec. 10, 1996 to Stanisiewski, an enhanced reproductive performance in cattle is achieved by commencing administration of growth hormone during the early postpartum period.
U.S. Pat. No. 5,637,566, issued on Jun. 10, 1997 to Walker et al. describes the use of growth hormone to improve carcass quality and/or food conversion efficiency in animals. In U.S. Pat. No. 4,792,546, issued on Dec. 20, 1988 to Baker, growth hormone is administered to food-producing animals to increase weight gain and at the same time reduce fat deposition and improve the lean meat to fat ratio.